The invention relates to a method for the pulsed magnetization of a high temperature superconductive (HTS) kryomagnet, which is operated below its transient temperature, and which consists of discs stacked along an axis and comprising each n annular or polygon-shaped concentric conductor elements of superconductive material with nxe2x88x921 annular gaps formed between the n concentric conductor elements.
HTS massive material capable of carrying high currents can be used for kryomagnets as long as, after the magnetization, it is maintained at an operating temperature T below the transient temperature Tc, that is, T less than Tc. Then the kryomagnet becomes, in effect, a permanent magnet. Its field is frozen. Fields of  greater than 14 Tesla have been demonstrated to exist after magnetization by large superconductive magnet coils by way of the xe2x80x9cField-Cooledxe2x80x9d procedure. The procedure is, in principle, as follows:
Within the initially time-wise constant outer field of for example a superconductive coil, the HTS is cooled to a temperature T less than Tc. At this temperature, the magnetic flux is frozen or captured. Then the outer magnetic field is reduced slowly that is on a scale of minutes and hours, whereby superconductive currents are induced in the HTS which substantially maintain the field in the HTS and which render the HTS in effect permanent magnetic that is, they form a kryomagnet.
The magnetization of HTS bodies capable of carrying high currents when installed in an electric machine cannot be performed by the use of a large superconductive coil but must be done by way of a pulsed magnetization using for example a Cu coil. In contrast to the above xe2x80x9cfield-cooledxe2x80x9d procedure the superconductor is cooled by the so-called xe2x80x9czero field-cooledxe2x80x9d process without outer field to a temperature  less than Tc and is then subjected to a short magnetic field pulse. With sufficiently strong magnetic fields, magnetic flux can be frozen in the superconductor also with this process. The magnetization may also occur in successive magnetization steps by multiple successive pulsing of the magnetizing magnet. Multipurpose processes with pulse durations of several ms have been found to be advantageous in this connection in order to freeze magnetic fields of up to 3 Tesla.
Pulsed magnetizing processes using CU coils without coupled current pulses [I, II, III] as well as shaping and connecting techniques for HTS solid material [IV], HTS ring structures and their magnetic characterization [V] as well as mechanical reinforcements for accommodating the high forces [VI] generated by the strong magnetic fields and effective on the HTS, are known.
The saturation magnetization of a shaped body, that is, the maximum field H* that can be frozen is determined by the shape of the sample and by the critical current density thereof. As a general rule, with the field-cooled method the field of the coil must be at least 1xc3x97H* in order to fully magnetize the probe. With a pulsed magnetization however, that is, the zero field-cooledxe2x80x9d procedure, typically a magnetic field of a pulse height 2xc3x97H* is necessary. The reason is that shielding currents are generated in the probe in the area of the rising flank of the magnetizing pulse,.
These shielding currents induced with the increasing flank of the magnetizing current pulse and the pulse fields of 3-6 Tesla maximally achievable with the installed Cu coils determine consequently the practical limits for frozen feeds maximally achievable.
If the induced shielding currents could be limited, ideally to zero, such a situation, which is comparable to the xe2x80x9cfield-cooledxe2x80x9d process would not be reached. If furthermore the individual shaped bodies could be separately magnetized the fields generated by the individual segments would add up and, altogether fields could be obtained which are higher than those generated by the Cu coil.
It is therefore the object of the present invention to provide a magnetizing procedure for a kryo-HTS magnet whereby high magnetic fields can be frozen at temperatures below the transient temperature Tc and to provide a kryomagnet which can be effectively magnetized in by this procedure.
In a method and a kryomagnet for the pulsed magnetization of the kryomagnet which comprises discs stacked on top of one another, with each disc including concentric annular conductor elements arranged in axially spaced relationship and each conductor element having two contact points forming two arms between the contact points for their energization, a transport current impulse is applied to each conductor element which pulse is divided in each conductor element into first and second partial currents I1 and I2 to flow through the two arms from one of the contact points, in an opposite sense, to the other contact point, wherein one arm has a length of maximally 35% of the circumference of the conductor element, the transport current having a polarity such that the larger partial current flowing through the shorter arm while the transport current is increasing flows in all the conductor elements in the same direction.
For a better understanding of the method, first the design specifically of the kryomagnet is shortly, described: The kryomagnet consists of m discs in a stack with the centers of the dics being all disposed on an axis. Each disc comprises n circular or polygonal conductor elements which are disposed concentrically in a plane and form therebetween nxe2x88x921 annular gaps, m and n being natural numbers xe2x89xa71. The conductor elements consist of superconductive, or more specifically, high temperature superconductive material.
Each of the n conductor elements has two contact points by way of which it is energized during the magnetizing procedure below the lowest transient temperature Tc of the respective used superconductive materials.
To each of the n conductor elements, a transport current impulse I pulse of a predetermined polarity strength and pulse form is supplied by way of its two contact points. From one contact point to the other of an energized conductor element, the transport current I pulse is separated into two partial currents I1 through one arm of the conductor element to the other contact point and I2 through the other contact arm of the conductor element to the other contact point. The two contact points are so arranged that the length of the connecting path therebetween that is, the length of the shorter of the two arms, comprises maximally 35% of the total circumference of the conductor element. In this way, a current asymmetry I1xe2x89xa0I2 is established. The current flowing in the longer arm will be designated I2.
The m, n conductor elements are geometrically arranged and electrically so interconnected that the transport current impulse Ipuls introduced into each of the n conductor elements has such a polarity that the partial current I1 has, in the area of the rising flank of the transport current impulse Ipulse, with respect to a predetermined sense the same direction in all n conductor elements. With the use of several discs, the transport current impulse Ipulse supplied to the conductor elements is so selected that the partial current I1 flowing during the increasing flank of the transport current pulse Ipuls has, with respect to a predetermined sense, in all discs the same direction.
Preferably, the transport current impulse is adjusted in all m, n conductor elements in such a way that the respective maximum value Ipuls,mzx is the same in each conductor element. The largest part of the length of the shorter arm of the full circumference of the closed conductor loop is designated for all m, n conductor elements with Amax. As critical current Ic of a superconductive conductor element, the current is designated which generates in the superconductor a voltage drop of 10xe2x88x926 V/cm. Currents  greater than Ic lead to a buildup of an ohmic resistance in the superconductor. The largest critical current of all the m, n conductor elements is designated Ic,max and the magnetic field strength, which is generated by all m fully magnetized discs in the centers thereof is designated H*. Accordingly, the maximum value Ipuls, max of the transport current impulse Ipuls is so adjusted that the following condition if fulfilled:
(1xe2x88x922Amax)Ipuls,maxH*/IC,maxxe2x89xa72H* 
The highest saturation magnetization is achieved in that by multiple repetition of the pulsed magnetizing procedure the remanent magnetic flux introduced into the kryomagnet is increased stepwise up to the saturation magnetization.
After each magnetization step, the operating temperature T is preferably reduced. This reduces the shielding effect especially of the conductor elements arranged on the outside and provides for an increased magnetization in the center of the kryomagnet (see reference III).
For the magnetization procedure described so far, no outer magnetic field generated by a copper coil installed in the system is necessary.
However, an outer magnetic field can be used for the magnetization. To this end, the kryomagnet system should include at least one copper coil. The axis of the outer magnetic field generated thereby coincides with the axis of the magnetic field, which has been frozen after magnetization.
The kryomagnet is exposed by way of the normally conductive coil to a magnetic field pulse Hpuls of predetermined polarity, strength and pulse form, which induces in each of the conductor elements an annular current Iind. This shields the conductor element at least partially from the intrusion of the magnetic flux during the increasing pulse flank of the magnetic field. After reaching the maximum Hpuls, max the polarity of the induced annular current Iind reverses.
By way of the two contact points, additionally, a transport current impulse Ipuls of predetermined polarity, strength and pulse form is applied to the respective conductor elements, which impulse is divided into two partial currents upon entering the conductor element.
Polarity, strength, pulse form and time-succession of the two pulses Ipuls and Hpuls are so selected that their cooperation results in a current distribution I1xe2x89xa0I2 in the two arms of the annular conductor element. Below, the partial current, which results from the cooperation of the two currents Ipuls and Iind and which has the same polarity as the annular current Iind which has been induced during the increase of the magnet pulse flank, is designated I1. This partial current I1 is, during the rising impulse flank, larger than the partial current I2, which flows in the other arm of the annular conductor element.
The magnetic field pulse Hpuls and the transport current pulse Ipuls are so selected that, during a time interval within the total pulse interval, at least the partial current I1 is in the vicinity of the critical current Ic of the respective conductor element or exceeds the critical current Ic. In this way, a higher ohmic resistance is built up which limits in the respective conductor element the maximum current and consequently reduces the shielding effect of the ring current Iind induced during the increasing pulse phase. As a result, the magnetic flux intrudes more strongly into the conductor loop and, after fading of the two pulses Ipula and Hpuls, a permanent superconductor current continues to flow in the conductor loop. In this way, a higher remanent magnetization is obtained than with the sole application of the magnetic field impulse Hpuls.
With respect to the transport current impulse Ipuls, the same conditions are set as during the magnetizing without outer magnetic field, that is, the magnetic field Hpuls and the transport current impulse Ipuls supplied to the mn conductor elements are so selected that the greater partial current I1 flowing during the increasing flank of the transport current Ipuls has the same direction in all conductor elements and, consequently in all n discs.
The assymetry in the division of the currents I1 and I2 in a conductor element is determined by the different arm lengths. The polarity of the current impulse Ipuls is so selected that, during the rising flank of the current impulse Ipuls, the larger partial current I1 flows in the shorter arm.
Preferably in all mn conductor elements of all the m discs the transport current impulse Ipuls is so adjusted that the respective maximum value Ipuls,max of the magnetic field pulse Hpuls, the largest part Amax of all the conductor elements of the length of the shorter arm of the total circumference of the closed conductor loop, the largest critical current Ic,max of all the mn conductor elements and the magnetic field strength H*, which is generated by all m fully magnetized discs in the centers thereof, are so selected that the following conditions are fulfilled:
Ipuls,max less than 2Ic,max 
and 
Hpuls,max+(1xe2x88x92Amax)Ipuls,maxIpuls,maxH*/Ic,maxxe2x89xa72H*. 
In a particular embodiment, the n conductor elements of one of the m discs are arranged in series with at least one copper coil. This makes it possible that the pulsed coil current or a part of the coil current can be used also as transport current pulse Ipuls in all n conductor elements. Depending on the dimensioning of the copper coil and the conductor cross-section of the n conductor elements, it may be necessary to limit the transport current impulse Ipuls, that is, therefore to supply only a part of the total coil current to the conductor elements.
It is possible, for example, to arrange the m discs electrically in a series circuit such that the pulsed coil current or a part of the coil current passes through all the m discs as transport current pulse Ipuls.
Preferably, the magnetic field pulse Hpuls and the transport current pulse Ipuls are generated by the discharge of a condenser into the coil arrangement. By way of a sufficiently fast electronic switch such as a thyristor or a power transistor the oscillation circuit of inductivity and capacity is divided at the predetermined point in time. In this way, only the first half of the developing natural oscillation is used for the magnetization and a fading out of the oscillation is prevented.
Preferably, the pulsed magnetization procedure is repeated multiple times whereby the remnant flux introduced into the kryomagnet is increased stepwise up to the saturation magnetization.
With this repeated pulsed magnetization, the operating temperature T is lowered with each magnetization step. As disclosed in the reference III, this procedure can be combined with lowered Hpuls,max values for the first magnetization pulses. The kryomagnet with which the magnetization is achieved only by its energization, comprises the following components:
It includes m stacked discs with a common axis. Each of the m stacked discs comprises n different concentric circular or polygon-shaped conductor elements of superconductive or, rather high temperature superconductive material, which are arranged concentrically in a plane. M and n are natural numbers each being xe2x89xa71. The number is determined by the technical application and the required magnetic properties of the kryomagnet.
Each conductor element of the kryomagnet has two contact points for its energization. The mn conductor elements consist of materials of the so-called SE, Ba2Cu3Ox high temperature superconductors, in short, 123-HTS. SE represents the chemical element Y or a rare earth metal or a mixture thereof. Each conductor element may include chemical additives, which increase the current conducting capacity. The crystallographic c-axis of the 123-HTS material of each of the conductor elements of a disc is displaced maximally 10 degree from the axis of the disc.
The conductor elements may comprise one or several 123-HTS shaped bodies. With the use of several shaped bodies, the bodies are joined mechanically and superconductively by superconductive connections on the basis of a 123-HTS with low peritectic temperature. The crystallographic a-b lattice intersections of the 123-HTS and 123 HTS materials are turned in the disc plane relative to each other by maximally 10xc2x0.
A plurality of electric circuit arrangements may be utilized. For example, the m n conductor elements may be connected each separately to an electric power source. Or the n conductor elements of a disc are arranged in a series circuit with the electrical connections for the supply and return line connected to the outer and inner rings, respectively. The electrical connection between the conductor elements may be normally conductive or superconductive. With a stack of discs, the discs may be separately connected or they may be arranged in a series circuit.
The inclusion of a normally conductive coil in the HTS kryomagnet for generating the outer magnetic field will be explained below:
For the magnetizing process, it is, in principle, unimportant what type of normally conductive coil is used as long as the field geometry of coincidence of the kryomagnet axis and of the external magnetic field axis is established. A copper coil is technically most suitable based on the material and manufacturing properties.
Two types of coils may be used, that is, a solenoid extending at least partially around the kryomagnet, that is the stack of the n discs, and a planar spiral coil of copper with a maximum diameter corresponding to the that of the discs.
In an arrangement, which is advantageous with regard to material stresses, the HTS kryomagnet is disposed in a matrix consisting of wax or resin or epoxy or another polymer hydrocarbon compound which is suitable for the kryo requirements and still has plastic properties at such low temperatures. In this way, the mechanical stresses associated the magnetic fields can be at least partially compensated and the mechanical stresses on the HTS material or reduced.
The pulsed magnetizing procedure proposed herein and the corresponding kryomagnet design have the following advantages:
In the state of the art massive bodies or also annular conductor structures on the basis of 123-HTS are used which are magnetized by large superconductive magnetic coils or also by pulsed copper magnet coils. By the direct supply of transport currents Ipuls to the various conductor elements, higher frozen magnetic fields as compared to the prior art can be obtained with a cost-effective and space-saving pulsed magnetization.
The invention will be described below in greater detail on the basis of the accompanying drawings.